Intussusception
- Intussusception is when one segment of bowel telescopes inside the next, like a collapsible camping cup folding into itself.
- It is the classic cause of bowel obstruction in young children — think toddlers, not teenagers — and most are ileocolic.
- Ultrasound is the test of choice: look for the "target" sign in cross-section and the "pseudokidney" in long axis.
- The fix is usually a non-surgical air (or contrast) enema — radiology doesn't just diagnose this one, it treats it.
- Free air on the films means a perforation: stop, call surgery, do not reduce.
Take a collapsible travel cup — the kind that telescopes down into a flat disc — and imagine it doing that inside your own gut. One stretch of bowel burrows headfirst into the segment just downstream and gets swallowed up. That, in one slightly unsettling image, is intussusception. The inner tube (the part that invaginates) drags its own blood supply in with it, and that's where the trouble starts.
Why it matters
When bowel telescopes, two bad things happen at once. First, the lumen gets plugged, so you get a bowel obstruction. Second — and this is the urgent part — the dragged-in mesentery gets pinched, the veins back up, the wall swells, and if you wait long enough the blood supply chokes off entirely and the bowel dies. So it's a clock-is-ticking problem, not a come-back-next-week one.
It's the most common cause of bowel obstruction in kids roughly between 3 months and 3 years, and the classic story is a previously happy toddler who pulls their knees up in waves of colicky pain, then goes limp and pale in between. The textbook "currant jelly stool" (blood and mucus) is a late sign — by the time you see it, the bowel is already unhappy, so don't wait for it.
Most childhood cases are idiopathic — no underlying mass, just enlarged lymphoid tissue in the bowel wall acting as a speed bump that the next peristaltic wave grabs onto. The vast majority are ileocolic: small bowel telescoping into the colon near the right lower quadrant.
What you actually see
Start with plain films, mostly to make sure you're not about to reduce a perforated bowel. They can be normal, or show a soft-tissue mass and a paucity of gas in the right lower quadrant where the colon should be. Films are the bouncer at the door, not the main event.
The main event is ultrasound, and it's genuinely satisfying. Cross the probe over the telescoped segment and you get the target (or doughnut) sign: concentric rings of bowel wall stacked inside each other, alternating bright and dark, like the cross-section of a tree trunk that someone stuffed extra rings into. Rotate to look along its length and the same blob looks like a kidney — the pseudokidney sign — because the layered wall mimics renal cortex and the dragged-in fat mimics the fatty hilum.
CT isn't the first-line test in a small child — we'd rather not hand out radiation dose we don't need — but you'll bump into intussusception incidentally on adult and older-child scans, where it shows the same bowel-within-bowel layered "sausage" with central fat.
Reduction: where radiology does the fixing
Here's the part students love: in the right patient, this is treated in the radiology suite, no scalpel. Under fluoroscopic or ultrasound guidance, we push air (or sometimes water-soluble or barium contrast) retrograde up through the rectum and let the pressure shove the telescoped segment back out the way it came — like blowing the collapsed travel cup back open.
Air enema reduction is the modern workhorse: it's clean, it's quick, and if the bowel does perforate during the attempt, air in the belly is far easier to manage than spilled barium. The radiologist watches the column of air march around the colon and pop the intussusception free.
But reduction has hard rules. Don't attempt it if the child has peritonitis, is in shock, or there's free air (perforation) on the films — those go straight to the operating room. Reduction can also fail, or the thing can come back, so a surgeon should always be in the loop before you start.
Beware the lead point. In an idiopathic case there's nothing to grab onto, but outside the typical toddler age — older kids, or a recurring case — a real mass (a polyp, a Meckel diverticulum, a duplication cyst, enlarged nodes from lymphoma) can act as the head of the telescope. If the patient is older than usual or it keeps recurring, hunt for the lead point rather than just reducing and shrugging.
Don't confuse it with the other emergency
One more trap: intussusception is colicky and (in the classic case) draggable back into place, whereas malrotation with midgut volvulus is the bilious-vomiting, twist-and-strangle catastrophe of the newborn. Different age, different bowel, different fix. If a young baby is vomiting green, that's a different page and a different kind of urgent.
The one thing to remember
Telescoped bowel, in a toddler, with a target sign on ultrasound — and unless there's a sign of perforation, the same department that found it can usually fix it with a puff of air.